While it is well-established that the endoplasmic reticulum (ER) plays a critical role in cellular homeostasis, its significant contribution to apoptosis is only now becoming apparent. This proposal aims at discovering the molecular mechanisms of the ER stress-induced apoptosis and of the antiapoptotic function of ER chaperone protein GRP78/BJP. We hypothesize that the ER stress-induced apoptosis initiates from the ER but is amplified through the mitochondria via multiple pathways and that one anti-apoptotic function of GRP78 relies on its prevention of activation of pro-apoptotic components that initiate the cell death program from the ER. Our hypothesis is based on the recent discovery that the ER is a site of convergence and regulation of both pro- and anti-apoptotic components, and that GRP78, a key regulator of the unfolded protein response, can protect cells against apoptosis induced by ER stress as well as DNA-damaging topoisomerase inhibitors. Further, GRP78 can exist as a transmembrane protein and interact with caspases inducible by ER stress or etoposide and block their activation. Towards understanding the underlying in vivo molecular mechanisms, we have three specific aims. In Aim 1, through the use of Apaf-1 deficient and BAX/BAK double knock MEFs, we will assess the requirement of the mitochondrial branch for ER-initiated apoptotic pathways, the relationship between known ER apoptotic pathways and ER BAK/BAX activation. We will further identify the molecules linking ER stress to activation of mitochondrial apoptotic pathway and their induction mechanism by ER stress. In Aim 2, we will identify steps of the ER-stress and etoposide-induced apoptotic pathway that is suppressed by GRP78 and test whether GRP78 is a novel inhibitor of BIK, an upstream regulator of BAX. In Aim 3, we will determine whether the ER stress-induced apoptotic pathways are altered in cancer, and the cytoprotective function of GRP78 in amyloid-beta toxicity in neuroblastoma and endothelial cells within tumors. The proposed work will not only contribute novel information on basic cell biology but also has clinical relevance in eliminating drug-resistant cancers and neurodegeneration in patients.